Current regulating circuits employing light sensitive means and pulse width modulation control



Oct. 5, 1965 DYKE 3,210,647

J. R. CURRENT REGULATING CIRCUITS EMPLOYING LIGHT SENSITIVE MEANS AND PULSE WIDTH MODULATION CONTROL Filed March 13, 1962 QC. AM T2 5 INVENTQR Joy/v P0051? Dv/(e' MWW ATTORN EYS United States Patent 3,210,647 CURRENT REGULATING CIRCUITS EMPLOYING LIGHT SENSITIVE MEANS AND PULSE WlDTH MODULATION CONTROL John Roger Dyke, Wembley, Middlesex, England, assignor, by mesnc assignments, to International Computers and Tabulators Limited, London, England, a British company Filed Mar. 13, 1962, Ser. No. 179,408 Claims priority, application Great Britain, Mar. 15, 1961, 9,464/61 Claims. (Cl. 32321) This invention relates to current regulating circuits.

It is the object of the invention to provide an arrangement for regulating the current flowing in a load which is supplied with uni-directional current containing a ripple frequency component.

According to the invention a current regulating arrangement includes a source of uni-directional current containing a ripple frequency component, a load device energized by the current source to produce a characteristic output, means for examining the output to produce a resultant signal which is representative of variations in the characteristic output, a source for supplying a reference signal, means for combining said resultant signal and said reference signal to produce a train of width-modulated control pulses at ripple frequency, the width of the pulses being varied according to the departure of the characteristic output from a required norm, and current control means connected to the current source to present an impedance switchable in response to the control pulses between a first and a second value to vary the mean current flowing through the load to counteract such departure.

In one form of the invention the load consists of a light source, and the means for producing the resultant signal includes a photoelectric cell which receives light from the source.

The invention will now be described, by way of example, with reference to the accompanying drawing, which shows an arrangement for regulating the current flowing through an incandescent lamp.

An incandescent lamp 1 forms a light source in a conventional arrangement for the photoelectric sensing of a punched record card. The light from the lamp is focussed on a column of the card by a suitable optical system. A line of photoelectric cells are positioned on the opposite side of the card so that light passing through a hole punched at a particular index point will fall on a corresponding one of the photoelectric cells. The card is fed step by step to bring all the columns of the card into sensing position in turn. It is desirable to maintain a uniform intensity of the light source to produce standardised outputs from the sensing cells.

Operating current for the lamp 1 is provided by a fullwave bridge rectifier, consisting of diodes 2, which is fed from the 50 c./s. mains supply via a transformer 3. No smoothing capacitor is employed, and the output of the rectifier therefore may be regarded as a uni-directional current containing a substantial ripple frequency component of 100 c./s. The current flows through the lamp 1 and through a control circuit consisting of a transistor T1 in parallel with a resistor 4. The impedance of the control circuit is equal to that of the resistor 4 if the transistor T1 is cut off. A typical value for the resistor 4 is three ohms for use with a lamp rated at 12 volts, 48 watts. If the transistor T1 is driven into full conduction, it passes the major part of the load current flowing through the lamp 1, and the effective impedance of the control circuit is greatly reduced. Consequently, the current through the lamp is substantially larger than with the transistor cut 01f. The average current flowing through the lamp is controlled by repeatedly switching the transistor on and off and varying the ratio between the durations of the on and off periods. This switching action is performed in a manner to be explained and is controlled by examination of the light output of the lamp, so that, for example, the mean lamp current is increased if the light output decreases.

The switching of T1 is carried out at the ripple frequency of 100 c./s., and it will be appreciated that the signal controlling the transistor T1 must be so phased, relative to the main supply, that the operation of the transistor in each cycle does not accentuate the ripple component of the load current. Thus, the light output of the lamp 1 contains a similar ripple component to that of the energising current. However, because the thermal time constant of the filament of the lamp 1 is several times greater than the period of the ripple frequency, the ripple component in the light output is displaced in phase relative to the ripple component of the rectifier output voltage. In a typical case this phase displacement is of the order of and the amplitude of the ripple frequency component in the light output is of the order of one percent of the mean light output.

Examination of the light output is performed by a monitor photo-voltaic cell 5 positioned to receive unobscured light from the lamp 1. The output from the cell 5 will be a uni-directional current, the magnitude of which is determined by the light output of the lamp 1, together with the small ripple frequency component referred to above. Hence, this current may be compared with a reference current to produce a difference signal which is indicative of any variation of the light output of the lamp.

The reference current is provided by a Zener diode 6 in conjunction with high stability resistors 7 and 8. The Zener diode 6 is connected in series with a resistor 9 between ground and a +12 volt supply line 10. The potential of the junction of the Zener diode and the resistance 10 is fixed by the diode, so that a known current flows through the resistors 7 and 8. This current may be set to a suitable value by adjustment of the variable resistor 7.

The difference betwen the reference current and the current of the cell 5 flows from the base of a transistor T2, the emitter of which is connected to ground. A diode 11 protects the transistor against excessive reverse base-emitter voltages. The collector of the transistor is connected to a 6 volt supply line 12 through a resistor 13, and to the positive line 10 through resistors 14 and 15. The junction of the resistors 14 and 15 is connected to ground through a diode and to the input of a multi-stage transistor D.C. amplifier 17 so that the amplifier receives an input representative of the difference between the current from the photocell 5 and the reference current.

The amplifier 17 is required to produce an output consisting of current pulses at ripple frequency to control switching of the transistor T1 and is built up of a chain of direct-coupled switching transistor stages so arranged that each stage switches a larger current than the preceding stage. Each stage has a transistor with its emitter connected to earth and its collector connected through a resistor to the 6 volt supply line 12. The output from the collector of each stage is directly connected to the base of the transistor of the next stage of the chain, and the output from the collector of the final stage is directly connected to the base of the transistor T1.

The reference current derived from the Zener diode 6 circuit is adjusted so that under normal operating conditions, that is when the required mean value of current is flowing in the lamp 1, the ripple frequency component only of the output from the photo-voltaic cell is amplified by the transistor 2 and this amplified signal appears at the junction of resistors 13 and 14. The effect of the resistors 13, 14 and 15 is to bias the amplifier 17 substantially about the mean of this signal, which is approximately sinusoidal. Thus, under normal conditions the amplifier 17 produces an approximately square-wave output in which pulses occur-at ripple frequency and correspond in width substantially to the inter-pulse periods.

These pulses are applied to the base of the transistor T1 to switch the transistor on. Thus, the transistor T1 is turned on for a period during each cycle of the effectively 100 c.p.s. supply to the lamp 1.

The operation of the apparatus under normal conditions may now be briefly summarised. The lamp 1 is energised by a DC. supply which has a ripple component and is connected in series with an impedance which is switched between two values at a frequency corresponding to the frequency of the ripple component. Taking each ripple cycle as being a duty cycle it will be seen that the impedance is set at each of its two values for approximately the same time in each duty cycle during normal operation. In other words, the impedance is switched from its higher value to its lower value for about 50% of each duty cycle, and this switching is controlled by switching on the transistor T1. The transistor is switched in this way as the result of examining the light output of the lamp 1. Because the lamp is energised by a current having a ripple component, the light output also has a similar component and so does the output of the photocell 5 which examines the light. The photocell output is first applied to a difference circuit to remove the DC. pedestal and the resultant signal from the transistor T2 (which is effectively only the ripple component) is then applied to an amplifier which is biased about the mean level of the resultant signal to provide a square waveform output in which control pulses occur each having a duration of approximately 50% of the duty cycle. It is these pulses which are applied to switch on the transistor T1.

Thus, if the light output of the lamp decreases, the

will also decrease and the level of the resultant signal from the transistor T2 will rise relative to the effective biasing level of the amplifier 17. Hence the output control pulses from the amplifier will ecah increase slightly in width so that the transistor T1 is switched on for fractionally longer than 50% in each duty cycle, with the result that the mean current flowing through the lamp is slightly increased and the required level of illumination is restored. Thus the difference between the two signals applied to the transistor T2 is effective to control the width of the output pulses from the amplifier 17 to counteract changes in the intensity of the light output of the lamp. It will be appreciated that this compensating effect is also true if the light output of the lamp increases, the control circuit then being effective to reduce the width of the output pulses from the amplifier 17.

The stability of the load current is largely determined by the stability of the reference source and the photocell 5. A high order of stability is provided by the use of a Zener diode and high stability resistors as the reference source, and by the use of a silicon photoell. The sensitivity of regulation is increased by decreasing the percentage of the ripple frequency component in the light output of the bulb. This can be achieved by providing some degree of smoothing for the rectified voltage. The sensitivity can also be increased by increasing the value of the resistor 4, but this increases the switching loss in the transistor T1.

It will be appreciated that a stabilised light source is useful in such applications as film processing machinery, as well as in devices for sensing data recorded on cards, tape and the like. Furthermore, the load need not be the filament of a lamp. In such cases, the load current is examined by a resistor, for example, through which the load current passes instead of the photocell described above.

It will be appreciated that for maximum effectiveness of control, the transistor controlling the current through the load should be switched off at the time when the voltage from the input receifier is at its peak. Where the load and the device for examing the load current are both purely resistive, it wlil be seen that the phase relationships between the supply and the control signals are such that this ideal condition may be met. However, it has been found in practice that adequate regulation is obtained in the case described in detail where there is a phase displacement in light output due, as previously noted, to the long thermal time constant of the lamp.

The embodiment described is a series regulator system in which the variable impedance control circuit is in series with the load. It is also possible to apply the invention to shunt regulation systems in which the control circuit is connected in parallel with the load. This is suitable if the power supply source has a relatively high impedance. It will be appreciated that the control circuit then consists of a resistor in series with the transistor instead of in parallel, and that the amplifier 17 has to provide an output signal such that the mean level falls for a fall in the load current. This is conveniently arranged, for example, by providing an additional stage in the amplify! ing chain to invert the effective difference signal.

What is claimed is:

1. A current regulating arrangement including a source of unidirectional electrical current containing a ripple frequency component; a light source energised by said electrical current to produce a light output having a ripple component of similar frequency; means for examining the light output to produce a resultant electrical signal representative of the intensity of said light output including said ripple component; a reference signal source; signal combining means responsive to said resultant signal and to said reference signal to produce a train of Widthmodulated control pulses at said ripple frequency; said signal combining means being effective to modify the width of said control pulses in dependence upon varia? tions in the mean intensity of said light output; and current control means connected to the signal combining means, the unidirectional current source and to the light source, said current control means having an impedance switchable between a first and a second value in response to said control pulses to regulate the mean current flowing in the light source in accordance with variations in said mean intensity of said light output.

2. A current regulating arrangement including a source of unidirectional electrical current containing a ripple frequency component; an incandescent electric lamp energised by said electrical current to produce a light output having a ripple component of similar frequency; a photoelectric cell responsive to the unobscured light output of the lamp to produce a resultant electrical signal representative of the intensity of said light output including said ripple component; a reference signal source; a difference circuit responsive to said result-ant signal and to said reference signal to produce an output signal repre sentative of substantially only said ripple component; signal amplifying means responsive to said output signal to produce a train of width modulated control pulses at said ripple frequency, said amplifying means being effective to modify the widths of said control pulses in dependence upon variations in the mean intensity of said light output; and current control means connected to said signal amplifying means, the unidirectional current source and to the lamp, said current control means having an impedance switchable between a first and a second value in response to said control pulses to regulate the mean current flowing in the lamp in accordance with variations in said mean intensity of said light output.

3. A current regulating arrangement including a source of unidirectional electrical current containing a ripple frequency component, an incandescent electric lamp energised by said electrical current to produce a light output having a ripple component of similar frequency; a photovoltaic cell responsive to the unobscured light output of the lamp to produce a resultant unidirectional electrical signal representative of the intensity of said light output including said ripple component; a source of a reference voltage of sign and amplitude such that the difference between said resultant signal and said reference voltage is normally representative of substantially only said ripple component; means for combining said reference voltage and said resultant signal to produce an output representative of said difference; signal amplifying means responsive to said output signal to produce a train of Widthmodulated control pulses at said ripple frequency, said amplifying means being effective to modify the width of said control'pulses in dependence upon variations in the mean intensity of said light output; and current control means connected to said signal amplifying means, the unidirectional current source and to the lamp, said current control means having an impedance switchable between a first and a second value in response to said control pulses to regulate the mean current flowing in the lamp in accordance with variations in said mean intensity of said light output.

4. A current regulating arrangement including a source of unidirectional electrical current containing a ripple frequency component; an incandescent electric lamp energised by said electrical current to produce a light output having a ripple component of similar frequency; a photovoltaic cell responsive to the unobscured light output of the lamp to produce a resultant unidirectional electrical signal representative of said light output including said ripple component; a source of a reference voltage of sign and amplitude such that the difference between said resultant signal and said reference voltage is normally representative of substantially only said ripple component; means for combining said reference voltage and said resultant signal to produce an output signal representative of said difference; a signal amplifier comprising a succession of DC. coupled amplifying stages; means for applying said output signal to the signal amplifier; means for biasing the signal amplifier to produce in response to said output signal a train of control pulses in which the pulse periods are normally substantially equal in width to the inter-pulse periods, the biasing level being such that a change in the difference between the resultant signal and the reference signal produced in response to a fall in the mean light output of the lamp causes the consequent shift of the mean level of said output signal relative to the biasing level to increase the width of the control pulse periods relative to the inter-pulse periods; and current control means connected to said signal amplifier, the unidirectional current source and to the lamp,

said current control means having an impedance switchable between a first and a second value in response to said control pulses and being effective to increase the mean current flowing in the lamp in response to an increase in width of the control pulse periods.

5. A current regulating arrangement including a source of unidirectional electrical current containing a ripple frequency component; an incandescent electric lamp energised by said electrical current to produce a light output having a ripple component of similar frequency; a photovoltaic cell responsive to the unobscured light output of the lamp to produce a resultant unidirectional electrical signal representative of said light output including said ripple component; a source of a reference voltage of sign and amplitude such that the difference between said resultant signal and said reference voltage is normally representative of substantially only said ripple component; means for combining said reference voltage and said resultant signal to produce an output signal representative of said difference; a signal amplifier comprising a succession of D.C. coupled amplifying stages; means for applying said output signal to the signal amplifier; means for biasing the signal amplifier to produce in response to said output signal a train of control pulses in which the pulse periods are normally substantially equal in width to the interpulse periods, the biasing level being such that a change in the difference between the resultant signal and the reference signal produced in response to a fall in the mean light output of the lamp causes the consequent shift of the mean level of said output signal relative to the biasing level to increase the width of the control pulse periods relative to the interpulse periods; current control means connected in series with the lamp comprising a resistor shunted by a transistor; and means for applying the control pulses to the transistor, the transistor being rendered conductive only in response thereto to reduce the value of the impedance of said combination; whereby the mean current flowing in the lamp is increased in response to an increase in the width of the control pulse periods.

References Cited by the Examiner UNITED STATES PATENTS 2,832,034 4/58 Lilienstein et al. 323--22 2,896,149 7/59 Lowry et a1. 32228 2,996,653 8/61 Roof 32225 3,044,006 7/62 Barnard 322-28 LLOYD MCCOLLUM, Primary Examiner. ROBERT C. SIMS, Examiner. 

1. A CURRENT REGULATING ARRANGEMENT INCLUDING A SOURCE OF UNIDIRECTIONAL ELECTRICAL CURRENT CONTAINING A RIPPLE FREQUENCY COMPONENT; A LIGHT SOURCE ENERGIZED BY SAID ELECTRICAL CURRENT TO PRODUCE A LIGHT OUTPUT HAVING A RIPPLE COMPONENT OF SIMILAR FREQUENCY; MEANS FOR EXAMINING THE LIGHT OUTPUT TO PRODUCE A RESULTANT ELECTRICAL SIGNAL REPRESENTATIVE OF THE INTENSITY OF SAID LIGHT OUTPUT INCLUDING SAID RIPPLE COMPONENT; A REFERENCE SIGNAL SOURCE; SIGNAL COMBINING MEANS RESPONSIVE TO SAID RESULTANT SIGNAL AND TO SAID REFERENCE SIGNAL TO PRODUCE A TRAIN OF WIDTHMODULATED CONTROL PULSES AT SAID RIPPLE FREQUENCY; SAID SIGNAL COMBINATING MEANS BEING EFFECTIVE TO MODIFY THE WIDTH OF SAID CONTROL PULSES IN DEPENDENCE UPON VARIATIONS IN THE MEANS INTENSITY OF SAID LIGHT OUTPUT; AND CURRENT CONTROL MEANS CONNECTED TO SIGNAL COMBINING MEANS, THE UNDIRECTIONAL CURRENT SOURCE AND TO THE LIGHT SOURCE, SAID CURRENT CONTROL MEANS HAVING AN IMPEDANCE SWITCHABLE BETWEEN A FIRST AND A SECOND VLAUE IN RESPONSE TO SAID CONTROL PULSES TO REGULATE THE MEANS CURRENT FLOWING IN THE LIGHT SOURCE IN ACCORDANCE WITH VARIATIONS IN SAID MEANS INTENSITY OF SAID LIGHT OUTPUT. 